Raw coal is mined from the ground and is processed to provide a desirable commercial product. Raw coal includes a certain amount of gangue mineral content which, following combustion under standard condition, leaves a solid ash residue. Saleable coal most preferably has a fixed ash specification limit which is normally specified in contractual agreements between the producer and the purchaser. A typical example of an ash specification for high quality coking coal is 10%. If the ash level of produced coal increases above this level, the product may still be saleable but its price is deleteriously affected and/or some penalties for the producer may be incurred.
Raw coal after mining may be separated into a particular particle size by a screen mesh type or other classification-type device to separate the raw coal into predetermined particle sizes defined by, for example, the screen aperture size of the screen separator.
The separated coal of the desired size is then supplied to a processor, in this example a dense medium separator. There are a number of different dense medium separators currently in use depending on the size of particles being treated. For example, large lumps may be processed in heavy medium drums, heavy medium baths, heavy medium vessels, larcodems etc, and smaller but still coarse particles may be processed in heavy medium cyclones, heavy medium cycloids etc. Note that the words “heavy” and “dense” can be used interchangeably in this context. These types of heavy medium devices use a benign or inert finely ground powder of medium solids (such as magnetite or ferro-silicon) slurried in water to form a dense medium whose density can be controlled by the proportion of solids in the slurry. Mixing the raw coal with the dense medium enables separation on the basis of its density relative to the density of the dense medium. For example, coal with an ash level of 10% may be separable from higher ash components of the raw coal by adding the raw coal to a dense medium with a density of, for example, 1400 kg/m3. In this example, the 10% ash product coal might float clear of the higher ash material which might tend to sink in the dense medium. The material that floats would report to the overflow outlet of a separator and that which sinks would report to the underflow outlet.
For the specific case of a dense medium cyclone, it is separating efficiency of the coal particles that is often critical to maximising recovery and grade. The accepted industry standard for measuring efficiency is the partition coefficient curve with its characteristic D50 and Ep parameters. The D50 is the separating density of the particles and the Ep is a measure of the sharpness of the separation (a higher value of Ep indicates more misplacement of particles and hence a lower efficiency). Whilst the D50 of a separation is strongly related to the medium density, there are machine effects that lead to, almost invariably, the D50 being a little higher than the medium density. The extent to which it is greater is dependent on a number of parameters, including, but not limited to, medium density, medium stability, dense medium cyclone pressure, raw coal feed rate, degree of cyclone wave, and variations therein. The overall sharpness of separation is a strong function of variations in each of these parameters.
Currently partition curves for dense medium cyclones (DMCs), and hence DMC circuit efficiency, are obtained from laboratory washability analysis or tracer testwork at the plant. The data obtained has a degree of accuracy on which the industry has based decisions about the performance level of the DMC and whether corrective actions are necessary. Unfortunately, these tests are characterised by their relatively high cost and long time frame before information becomes available. Infrequent checks find sub-optimal performance resulting in significant coal loss.
Thus, if a particular parameter of a particle can be determined more quickly, it therefore provides the ability for determining processor performance and, if necessary, more quickly taking remedial action to ensure that the processor is operating satisfactorily. The determination of a parameter of a particle is also useful information in environments totally unrelated to processor performance, such as sizing of material produced by crushes, degree of calcination of material, and the like.